Effects of pore morphology and topography on the rate of capillary imbibition in metal porous structures exposed to ambient air

Capillary imbibition of porous structures is crucial for capillary driven evaporative cooling systems that utilize capillary drive to transport working fluid. Even though the use of working fluids with high surface tension improve the capillary pumping pressure, larger cohesive forces could lead to larger contact angles, if the constituent porous surface renders low surface energy. This study investigates the influence of different pore morphologies and topographies on the wettability transition of several different pore structures when exposed to air, using deionized water as the working fluid. The droplet absorption rates of the porous structures were obtained over a period of 28 days. The samples with larger microscopic pore-scale roughness and lower porosity exhibited greater retention of their wettability over time. The Cu sample with the largest microscopic pore-scale roughness and the lowest porosity maintained its wettability throughout the study period, while the Cu foam, which had the smallest microscopic pore-scale roughness and the largest porosity, lost its wettability within 3 days. The formation of the highly stable oxide layer in Al porous structures restrict further contamination compared to the oxide layer formed on Cu porous structures. Findings provide insights into determining the appropriate grain structures and pore parameters required to minimize the wettability transition of porous metals when designing capillary driven evaporative cooling systems.